In suppressed ion chromatography a suppressor device is used to remove either anions or cations from the eluent thereby, reducing the eluent to a weakly dissociated form. The analyte ion is typically converted to a conductive form thereby allowing detection of the analyte against a low background. Different types of suppressors have evolved over the years. The early suppressor devices used chemical means for accomplishing suppression. Typically an acid or base regenerant supplied the regenerant ions. Other types of suppressors use electrolytically generated ions to achieve suppression. Representative suppressor devices included packed bed suppressors, fiber suppressors, flat membrane and based suppressors. The above devices are typically operated with the regenerant flowing countercurrent to the eluent stream. In one form of electrolytic suppression the suppressed effluent is recycled from the detector for use as the source of water required for the electrolytic water splitting reactions. This mode of operation was called the recycle mode. In another mode called the external water mode of operation, the suppressor regenerant channels are supplied with water from an external source that is electrolytically split thereby forming the regenerant required for the suppression reaction. In this mode the external reservoir is replenished with water. Typically this mode was recommended for solvent containing eluents or when there was a need for improving the detection limits since this mode lowered the background noise.
Different forms of flat membrane suppressors is described in U.S. Pat. No. 4,999,098. In one, the suppressor includes at least one regenerant compartment and one chromatographic effluent compartment separated by an ion exchange membrane sheet. The sheet allows transmembrane passage of ions of the same charge as its exchangeable ions. Flow from the effluent compartment is directed to a detector, such as an electrical conductivity detector, for detecting the resolved ionic species. A sandwich suppressor is also disclosed including a second membrane sheet opposite to the first membrane sheet and defining a second regenerant compartment. Spaced electrodes are disclosed in communication with both regenerant chambers along the length of the suppressor. By applying an electrical potential across the electrodes, there is an increase in the suppression capacity of the device. The patent discloses a regenerant solution (acid or base) flowing in the regenerant flow channels and supplied from a regenerant delivery source. The patent also discloses the possibility of using water to replace the regenerant solution in the electrodialytic mode.
One potential problem with ion chromatography or other analytical methods such as high performance liquid chromatography (HPLC) is when the sample liquid contains a matrix of one or more compounds of high ionic strength. For chromatography, the sample peaks may be obscured by large interfering peaks of a matrix ion of the matrix compounds. Also, the chromatographic results can be significant also because the matrix ion is of such high concentration that it becomes the major eluting ion, temporarily overriding the eluent.
A membrane suppressor device used in ion chromatography (e.g., of one of the general type set forth in U.S. Pat. No. 4,999,098) has been used as a pretreatment device on-line with subsequent chromatographic separation using ion chromatography. Pretreatment reduces the concentration of the matrix ions of acid or base matrices. This technique is useful for the analysis of anions and cations only when the sample matrix is basic or acidic, respectively. This is because the suppressor device is also an ion exchange device, cation exchange for anion analysis and anion exchange for cation analysis. For example, neutralization of a basic matrix for analyzing anions requires the removal of the cationic co-ion to the hydroxide ion and replacing with a hydronium ion to form water for the neutralization reaction. The removal and replacement occur at ion exchange sites in the pretreatment device.
For some samples, continuous membrane based suppressor pretreatment device operated in the chemical mode may have the required capacity to treat the matrix ion. However, it may produce an interfering blank (e.g., sulfate for anion analysis with sulfuric acid as the regenerant). This is due to potential leakage of the acid regenerant used to supply the continuous source of hydronium ion for the neutralization reaction across the membrane.
A commercial neutralizer product sold by Dionex Corporation under the trademark ASRN was used as a pretreatment device for analyzing anions in a base stream. The published maximum dynamic suppression capacity of the ASRN suppressor is about 200 ueqv/min or about 200 mM at a flow rate of 1 ml/min. While this capacity is acceptable for most samples, it is difficult to suppress high concentrations of acid or base sample using one pass of the sample particularly at larger sample volumes. The larger sample volume requirement stems from the need to be able to detect trace ions with high sensitivity.
U.S. Pat. No. 5,597,481 discloses one approach to suppressing high concentrations and large sample volumes of acid or base matrices by passing the sample multiple times through the suppressor eluent channel thereby utilizing the regenerated capacity of the suppressor device. Currents as high as 500 mA was used to maximize the suppressable concentration. As disclosed, the device is in the form of an electrochemical membrane device in which sample flows through a sample flow channel of the device separated from a regenerant flow channel by an ion exchange membrane preferentially permeable to the same charge as the matrix ions and including exchangeable ions of that one charge. The pretreatment device includes electrodes in electrical communication with the sample flow channel and regenerant flow channel. The ionic species in the pretreatment sample device are directed to an analytical system comprising means for separating the ionic species and detector means for detecting the separated ionic species. One form of the pretreatment device includes two ion exchange membranes defining the sample flow channel. Two regenerant channels are on either side of the sample flow channel and are separated by the ion exchange membranes. The electrodes are in these outside flow regenerant channels. In another embodiment, the disclosed apparatus includes ionic species concentration column disposed downstream of, and in communication with, the pretreatment means for collecting and concentrating the ionic species to be detected. After concentration, the ionic species are eluted from the concentration means and directed to the analytical system. In a further disclosed embodiment, for use with a sample for which the pretreatment device has insufficient capacity, a conduit was disclosed for recycling the liquid sample stream to the sample flow channel as many times as desired to accomplish pretreatment prior to flow to the analytical system.